As is known in the art, in many applications it is required to electrically connect electrical components in a chassis to elements outside the chassis. Such a connection is sometimes made using an electrical connector, as shown in FIGS. 1A and 1B having: an electrically conductive housing with a flange mounted to an outer conductive wall or bulkhead of the chassis, not shown; and, a signal pin passing through, but dielectrically insulated from, the electrically conductive wall of the housing. The signal pin passes through the outer electrically conductive wall of the chassis into an interior region of the chassis to make an electrical connection to an electrical component, not shown, within the chassis. When the electrical components are mounted to a printed circuit board, such components may be electrically interconnected through microwave transmission lines, such as, for example, microstrip transmission lines, stripline transmission lines, or coplanar waveguide transmission lines. More particularly, such transmission lines include: a dielectric structure; a signal strip conductor disposed on an upper surface of the dielectric structure; and a ground conductor disposed on the dielectric structure and insulated from the strip conductor. Thus, to make the connection between the electrical connector and the microwave transmission line, the electrically conductive housing is electrically connected to the ground conductor of the microwave transmission line and the signal pin has an end portion electrically connected to an upper portion of the signal strip conductor of the microwave transmission line. One such an arrangement is shown in FIGS. 1A, 1B, 11B′ 1C and 1C′ where the microwave transmission line is a microstrip transmission line and the electrical connector is a surface mount RF connector. Thus, the microstrip microwave transmission line includes: a signal strip conductor disposed on an upper surface of a dielectric structure of a printed circuit board; and a ground plane conductor disposed on the bottom surface of the dielectric structure. Here, electrically conductive vias (FIG. 1A) pass from the upper surface of the dielectric structure through the dielectric structure to the ground plane conductor. The connector includes: an electrically conductive housing having electrically conductive arms; a pair of upper arms are soldered, and hence electrically connected to the upper surface portions of the electrically conductive vias and a pair of lower arms are soldered, and hence electrically connected to the ground conductor on the bottom surface of the dielectric structure; and, a signal pin, dielectrically insulated from, the housing and disposed on, and electrically connected to, the end portion of the signal strip conductor, as shown in FIGS. 1B, 1C and 1C′; the other end of the signal strip conductor being connected to an electrical component, not shown. It should be understood that that other electrical components, not shown, are in the chassis and hence such components must be electrically shielded from any RF radiation that may propagate from a connector-to-microwave transmission line interconnect region (FIG. 1B) between the signal pin and signal strip conductor. However, as shown more clearly in FIGS. 1C and 1C′, the front face of the dielectric structure is not perfectly flat and hence air gaps (FIG. 1C′) result which can leak microwave energy. Further, as shown in FIGS. 1D and 1E, an electrically conductive shield, such as copper or aluminum is sometimes positioned over the microwave transmission line interconnect region. The electrically conductive shield is a self-supporting bridge-like structure with the mid-section, or top, suspended over, and hence insulated from, the soldered connector signal pin and signal strip conductor by an air gap and with the outer sides, or legs, of the electrically conductive shield connected to the upper arms of the conductor, as shown in FIGS. 1D and 1E, and therefore to the ground plane conductor through the conductive vias. It is noted that the top and legs of the electrically conductive shield have apertures or perforations passing through them as shown. Therefore, the electrically conductive shield does not inhibit RF from leaking out of the sides and therefore is only partially effective in providing RF shielding.
In some application, a bulkhead connector is used as shown in FIGS. 2A, 2B and 2C. It is noted that the printed circuit board in this example includes RF electrical components; here for example, Monolithic Microwave Integrated Circuits (MMICs) interconnected by microwave transmission lines, here stripline microwave transmission lines having a strip conductor separated from a pair of ground plane conductors by a dielectric structure of the printed circuit board. The bottom ground plane conductor is mounted to an electrical conductive layer, which may also be thermally conductive, for example a cold plate, or heat spreader, as shown. It is noted in the example shown that a cutout, or notch, is formed in the printed circuit board by removing an upper portion of the dielectric structure and portion of the upper ground plane conductor to expose an end portion of the transmission line strip conductor. In order to interface with external components, bulkhead connectors are provided, as shown. More particularly, the bulkhead connector is, in effect, a coaxial connector, which passes through the bulkhead and has an outer conductor mounted to the bulkhead with an inner conductor, or conductive pin, electrically insulated from the outer wall by suitable toroid shaped dielectric element. It is noted that the conductive pin has an end portion projecting outwardly from one side of the bulkhead for positioning on, and electrical connection with, an end portion of the strip conductor of the microwave transmission line. Thus, when the bulkhead and electrically conductive layer, with mounted printed circuit board, are mounted together as shown in FIG. 2C, because of manufacturing tolerances, there will typically be a gap in the connector-to-microwave transmission line interconnect region between the outer sidewall of the printed circuit board and the opposing outer sidewall of the bulkhead. In any event, the projecting portion of the connector pin and the portion of the transmission line, both exposed by the cutout, are soldered or otherwise affixed and electrically connected. However, RF radiation passing through the gap in the interconnect region may propagate outside the connector-to-microwave transmission line interconnect region and thereby interfere with neighboring electrical components.
One attempt used to solve this RF radiation problem is shown in FIGS. 3A-3E. Here, a piece of solid, electrically conductive metal shim stock, for example, Indium shim stock, is cut and placed against the side of the printed circuit board before bolting the bulkhead to the printed circuit board (FIGS. 3A and 3C) However, the metal shim stock does not have a perfectly flat face. (FIG. 3A) and therefore, when mounted to the bulkhead (FIGS. 3C and 3D) the metal shim does not create a perfect RF energy “seal” between the printed circuit board and bulkhead and because of the rough surface of the metal shim air gaps are formed as shown in FIG. 3D through which microwave energy may leak. In some cases the metal shim stock isn't applied before-hand, so the only solution is to cover the gap with dielectric tape having an upper surface coated with an electrically conductive material, such as copper, as shown in FIG. 3E. This method only keeps RF energy from radiating out of the top of the connector, leaving space or air gaps for RF energy to escape out the sides.